I keep forgetting to always keep in mind which population the study was conducted in. I'm not a post-menopausal woman with Osteoporosis, so the findings of this might not apply to me well or at all.

Anyway, in short they found:

↑ ucOC --> ↑ Adiponectin; ↓ Body weight & ↓ Fat

In the study women given PTH (Parathyroid hormone) saw increased levels of ucOC & women given Bisphosphonates saw decreased ucOC. This was interesting to me since a lot of patients take Bisphosphonates & I was unaware of the potential negative effects on body composition they might have.

They knew that there is an association between Vitamin K insufficiency & elevated ucOC:

↓ VitK --> ↑ ucOC

They wanted to see if supplementing Vitamin K (in this case K1 AKA Phylloquinone) would lower ucOC levels & it did. The idea here being:

↑ VitK --> ↓ ucOC --> ↓ Fracture risk; ↑ BMD

So it seems ucOC is not clear-cut. We may want higher levels from a body composition standpoint, but lower levels from a bone health standpoint. I'm a young male, so you might think bone health should be low on my list of priorities, but I came across an interesting fact the other day that up to 25% of men will suffer a Fracture due to Osteoporosis during their lifetime -- 1 in 4, not terribly good odds.

Keeping with the beneficial effects of Vitamin K on bone health, I found this:

Available evidence suggests that the effect of vitamin K2 on
mineralization by human periosteal osteoblasts is enhanced in the
presence of 1,25 dihydroxyvitamin D3 in vitro. The effect of vitamin K2
on BMD in ovariectomized rats is affected by the plasma
25-hydroxyvitamin D3 level in vivo, and is significant only when rats
are fed a diet containing vitamin D3. Based on this line of evidence,
combined treatment with alfacalcidol and menatetrenone for osteoporosis
is surmised to be more effective than treatment with menatetrenone
alone, and may have anabolic effects on osteoporotic bone.

In short, Vitamin D3 & Vitamin K2 are both good for bone health & potentiate the effects of one another.

This study was interesting to me because it pointed out the difference between BMD (Bone mineral density) & BMC (Bone mineral content). Looking at BMD alone may not provide enough information because:

Unlike BMC, DXA-BMD does not take into account the geometry (size,
thickness) of bone, which has an independent [positive] contribution to bone
strength and fracture risk.

In conclusion, although I am not a post-menopausal woman with Osteoporosis, bone health does sound appealing to me & I believe there are other benefits to supplementing Vitamin K2 as well. Plus, I love natto (纳豆). While Vitamin K2 might decrease ucOC levels which might have negative effects on body composition, I am not too sure about the strength of this correlation & where it stands compared to other factors such as overall diet, activity level, etc. I imagine the effect is not too large since my body composition is quite good despite regular intake of large amounts of this delicious fermented food product.